The present invention is directed toward fiber-reinforced plastic foam structures. More particularly, the present invention is concerned with the method and apparatus for preparing novel fiber-reinforced foamed articles having a predetermined distribution of fibers throughout a foamed article.
A wide variety of materials have been added to foamed compositions to alter the properties of the foams for any one of a number of different reasons. Typically, fibers are added to foam compositions to increase the mechanical strength of the foam.
In some instances, it is desirable to have randomly oriented fibers distributed throughout a foam article as uniformly as possible. In the past, substantially uniform distribution of fibers in foam articles has not been achieved in any simple and economic manner, especially at fiber loadings above about 5 weight percent. Typically, a mold is loaded with randomly dispersed fibers and then a foamable resin mixture is added. During foaming, however, a substantial amount of the fibers are displaced from their original position with the end result that the fibers are not uniformly dispersed throughout the foam article and, for this and other reasons, the desired improvement in properties is not obtained.
In another technique a fiber-reinforced foam is fabricated by impregnating a non-woven batting of fibers with a foamable mixture. This technique suffers from the disadvantage that it is virtually impossible to adequately impregnate all the spaces between the fibers of the batting with a foamable mixture, again resulting in article of relatively low mechanical strength.
Foam articles have also been reinforced with woven fibers by bonding sheets of fabric to the exterior surfaces of a cellular foam article or by foaming a foamable composition between sheets of fabric. The strength characteristics of these cellular laminates are limited to a considerable extent upon the quality of the bond between the core and skin.
Spray techniques in which a foamable resin and fibers are sprayed onto a surface or substrate are not suitable for fabricating articles of any practical thickness. Indeed, such techniques are not at all applicable to the formation of structural foams, i.e., foams with low density cores and high density integral skins.
Mention should also be made of attempts to add fiber reinforcements to foam compositions during "cream time," i.e., when the foam starts to rise. Samples prepared in this manner will not have a predetermined uniform distribution of the fibers throughout the foam. Basically, it is extremely difficult, if not virtually impossible, to get reasonable fiber loadings mixed with the foam composition during the cream time.
These and other deficiencies of the foregoing fabrication techniques are well known.